linux-kernel - Re: [RFC PATCH 5/6] implement per-cpu&per-domain state machine call

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Message-ID: <20120308195825.GC2412@linux.vnet.ibm.com>
Date:	Thu, 8 Mar 2012 11:58:25 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Lai Jiangshan <eag0628@...il.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	patches@...aro.org, tj@...nel.org
Subject: Re: [RFC PATCH 5/6] implement per-cpu&per-domain state machine
 call_srcu()

On Tue, Mar 06, 2012 at 04:34:53PM +0100, Peter Zijlstra wrote:
> On Tue, 2012-03-06 at 23:12 +0800, Lai Jiangshan wrote:
> > On Tue, Mar 6, 2012 at 7:16 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> > > On Tue, 2012-03-06 at 17:57 +0800, Lai Jiangshan wrote:
> > >>         srcu_head is bigger, it is worth, it provides more ability and simplify
> > >>         the srcu code.
> > >
> > > Dubious claim.. memory footprint of various data structures is deemed
> > > important. rcu_head is 16 bytes, srcu_head is 32 bytes. I think it would
> > > be real nice not to have two different callback structures and not grow
> > > them as large.
> > 
> > CC: tj@...nel.org
> > It could be better if workqueue also supports 2*sizeof(long) work callbacks.
> 
> That's going to be very painful if at all possible.
> 
> > I prefer ability/functionality a little more, it eases the caller's pain.
> > preemptible callbacks also eases the pressure of the whole system.
> > But I'm also ok if we limit the srcu-callbacks in softirq.
> 
> You don't have to use softirq, you could run a complete list from a
> single worklet. Just keep the single linked rcu_head list and enqueue a
> static (per-cpu) worker to process the entire list.

I like the idea of SRCU using rcu_head.  I am a little concerned about
what happens when there are lots of SRCU callbacks, but am willing to
wait to solve those problems until the situation arises.

But I guess I should ask...  Peter, what do you expect the maximum
call_srcu() rate to be in your use cases?  If tens of thousands are
possible, some adjustments will be needed.

							Thanx, Paul

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